In a motor vehicle, a pull condition can occur in which a driver of the vehicle has to input torque into the steering system in order to maintain a straight path. This condition can be caused by vehicle and road conditions and weather conditions such as a crosswind that require a force for the steering system to overcome, or by an imbalance in the steering system (such as if the valve is balanced in the null position and the torsion bar is on center but the valve spool is creating pressure in the system) which requires an input torque by the driver to correct.
U.S. Pat. No. 5,749,432 describes a variable effort steering system that has the ability to provide torque at the input shaft with no driver input. This patent shows an angle sensor at the top and bottom of the valve assembly that are used to provide a signal indicating a rotational angular position of the spool shaft and magnet ring in order to control the amount of current provided to the actuator to provide variable effort steering. This system, however, is lacking in diagnostic capabilities, particularly with respect to vehicle pull conditions, because it does not monitor steering output.